This invention relates to an analog-type electronic wristwatch structure that is capable of being manufactured at low cost. More particularly, this invention seeks to provide an analog-type electronic wristwatch structure which is simplified by adopting a synthetic resin material to form a main plate, which constitutes a principal base plate forming part of a timepiece movement, and by employing a flexible printed circuit board, having a thickness of from 60 to 300 microns, as a circuit board for forming an electronic circuit block.
Analog-type crystal wristwatches have achieved popularity with comparative rapidity in recent years. These wristwatches are composed of such components as a crystal oscillator circuit serving as a time base oscillator, a frequency divider circuit which divides a high-frequency time base signal, produced by the oscillator circuit, down to a low-frequency unit time signal, a driver circuit which produces drive signals upon receiving the unit time signal produced by the frequency divider circuit, an electro-mechanical transducer, such as a stepping motor, driven in response to the drive signal from the driver circuit, a mechanical transmission mechanism such as a wheel train driven by the electro-mechanical transducer, hands which are advanced in accordance with the operation of the mechanical transmission mechanism, time correction means for correcting the time displayed by the hands, and a power supply battery for supplying the oscillator circuit, frequency divider circuit, driver circuit and electro-mechanical transducer with electrical energy.
The conventional analog-type crystal wristwatches have a main plate which generally consists of a metal material, the main plate serving as a principal base plate forming part of the timepiece movement. In providing the main plate with a shape called for by its functional characteristics, therefore, it is necessary to rely upon an extremely time-consuming machining method to cut the plate. This is one major obstacle to reducing costs.
On the other hand, flexible printed circuit boards having a thickness of from 60 to 300 microns have come into use as circuit boards, enabling large quantities of electronic circuit blocks to be mounted in continuous fashion. Such circuit mounting techniques, referred to as mini-MOD or film-carrier mounting techniques, have undergone rapid development in recent years and now make it possible to manufacture electronic circuit blocks for timepieces at extremely low cost. When a flexible printed circuit board is employed as the circuit board in an analog-type electronic wristwatch, however, a special reinforcing arrangement is called for because the printed circuit board itself does not have adequate mechanical strength. Since such an arrangement raises cost, there has in fact been a growing tendency away from the use of the flexible printed circuit board. More specifically, in the case of wristwatches, there is always the danger of an externally applied impact when the wristwatch is being worn, owing to the very nature of the commodity, so that such a commodity must have impact resistance. It is obvious that preventing major accidents, such as damage to the electronic circuit block, will be difficult unless sufficient protection against externally applied impact is provided for the electrical elements mounted on the printed circuit board, and for portions where connections exist between these elements and the circuit board. A flexible printed circuit board can be fixed and protected against externally applied impacts or the like with relative ease in the case of a digital-type electronic wristwatch which does not have a miniature moving portion such as a wheel train, and which is so constructed as to enable the circuit board to be embraced from above and below, over a wide area, by means of the electro-optical display cell and such members as a display cell supporting member or element cover. In the case of the analog-type electronic wristwatch, however, providing such protection for the flexible printed circuit board and for the electrical elements mounted on the board entails more than just a special arrangement. It requires additionally that special consideration be given to a fixing structure in the conventional case where the main plate, comprising the principal base plate, is composed of a metallic material. This is because electrical short circuits must not be allowed to occur owing to contact between the main plate and the conductive pattern or electrical elements on the printed circuit board. There is also an additional structural requirement in order to effect electrical interconnection between other components, such as battery lead members, and portions of the electrically conductive pattern provided on the flexible printed circuit board. Since the flexible board has none of the rigidity necessary for bearing the contact pressing forces exerted by such members as battery lead members, special connector members and a fastening arrangement become essential to couple the board and other members together for the purpose of maintaining stable electrical connection between them.
Adopting the flexible printed circuit board is an extremely effective measure for the low-cost manufacture of the electronic circuit blocks per se. Nevertheless, when the flexible printed circuit board is employed as the circuit board for constructing the electronic circuit block in an analog-type electronic wristwatch in accordance with the prior art, special members for protection, reinforcement, insulation and fixing must added in order to protect the electronic circuit block, to prevent short circuits and to stabilize the electrical interconnection among members. When the total cost for the manufacture of the entire timepiece is considered, adopting the flexible printed circuit board is not found to be an effective expedient for reducing such cost.